In recent years, the technology of organic light-emitting diodes (OLEDs) has advanced considerably. The efficiency and lifetime of OLED devices have been improved dramatically and several kinds of OLED displays have been commercialized. OLEDs have many attractive features for display and general lighting applications. They have high brightness, high efficiency, a wide viewing angle, and quick response time. In addition, they can be fabricated by depositing or printing organic materials on a single substrate (e.g. glass substrate), and as such, make it possible to utilize the features of the substrate.
An OLED is a light emitting diode in which an emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes. Generally, at least one of these electrodes is transparent.
FIG. 1 shows a typical conventional OLED which is composed of a thin transparent anode 10, an organic layer stack 20 with a light emission zone (not shown), a highly refractive intermediate layer 30 deposited on a glass substrate 50, and a cathode layer 40. The organic molecules in den organic layer stack 20 are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. These materials have conductivity levels ranging from insulators to conductors, and therefore are considered organic semiconductors. The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors. During operation, a voltage is applied across the OLED such that the anode 10 is positive with respect to the cathode 40. As a result, a current of electrons with negative charge flows through the device from the cathode 40 to the anode 10, as electrons are injected into the LUMO at the cathode 40 and withdrawn from the HOMO at the anode 10. This latter process may also be described as the injection of holes into the HOMO. Electrostatic forces attract the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.
The preparation of medium to large area (small molecule) OLEDs, especially of the layer structure consisting of organic materials, is usually carried out by thermal evaporation in vacuum on a light transmitting substrate, e.g. float glass. Unfortunately typically about 50% of the light generated remains in the OLED layer stack 20 (guided modes), about 25% remain in the substrate 50 with low refractive index n and only 20-25% are coupled into air and can be used for lighting applications. This portion of light emitted into air can be increased by a number of measures by about 50% to about 36%, which is still too low for an efficient use of the OLED. A further improvement can be obtained, if a normal glass substrate with a optically thick high refractive index layer (e.g. n=1.8, matching the average index of the OLED layers or of the anode layer) below the OLED and an additional out-coupling structure near the interface is used. Such a solution may be obtained by a rough interface or structured surface 60 between the substrate parts with high refractive index n and low refractive index n. However, an additional roughening step is required, e.g., by grinding, sandblasting and after that rather time-consuming structural etching of the glass (float glass) with low refractive index n is needed.
Then, a step of depositing a smoothing layer (e.g., the intermediate layer 30 with high refractive index) on the structured surface follows, after the roughening and etching steps have been performed, wherein the intermediate layer 30 has a refractive index being larger than the refractive index of the substrate 50. The intermediate layer 30 can be deposited by using e.g. chemical vapor deposition (CVD), wherein grooves of the structured surface 60 are filled with a material having a refractive index being larger than the refractive index of the substrate. For instance, this material can be SiOxNy or Si-Nitride.